Goldfish, Carassius auratus, as an infection model for studying the pathogenesis of Edwardsiella piscicida

Expanding the Spectrum of Diseases and Disease Associations Caused by Edwardsiella tarda and Related Species

The genus Edwardsiella, previously residing in the family Enterobacteriaceae and now a member of the family Hafniaceae, is currently composed of five species, although the taxonomy of this genus is still unsettled. The genus can primarily be divided into two pathogenic groups: E. tarda strains are responsible for almost all human infections, and two other species (E. ictaluri, E. piscicida) cause diseases in fish. Human infections predominate in subtropical habitats of the world and in specific geospatial regions with gastrointestinal disease, bloodborne infections, and wound infections, the most common clinical presentations in decreasing order. Gastroenteritis can present in many different forms and mimic other intestinal disturbances. Chronic gastroenteritis is not uncommon. Septicemia is primarily found in persons with comorbid conditions including malignancies and liver disease. Mortality rates range from 9% to 28%. Most human infections are linked to one of several risk factors associated with freshwater or marine environments such as seafood consumption. In contrast, edwardsiellosis in fish is caused by two other species, in particular E. ictaluri. Both E. ictaluri and E. piscicida can cause massive outbreaks of disease in aquaculture systems worldwide, including enteric septicemia in channel catfish and tilapia. Collectively, these species are increasingly being recognized as important pathogens in clinical and veterinary medicine. This article highlights and provides a current perspective on the taxonomy, microbiology, epidemiology, and pathogenicity of this increasingly important group.

The pathway of Edwardsiella piscicida infecting Lateolabrax maculatus via the immersion bath

Edwardsiella piscicida, an infectious bacterium, causes great economic losses to the aquaculture industry. Immersion bath which is the closest way to how the fish infect bacterial pathogens in the natural environment is an effective route of artificial infection. In this study, the dynamic process of E. piscicida infection, in the spotted sea bass (Lateolabrax maculatus) was evaluated via the immersion bath. The results showed that soaking the spotted sea bass with 3 × 106  CFU mL-1 E. piscicida for 30 min could artificially induce edwardsiellosis. The higher culture temperature (28.5 ± 0.5°C) or the longer bath time (30 min) would lead to higher mortality of fish. E.piscicida first invaded the gill, then entered the blood circulation to infect the spleen and kidney, where it is colonized, and gradually multiplied in the liver and brain. Meanwhile, the fluorescence in situ hybridization showed that the localization of E. piscicida in the gill and foregut after the immersion challenge proceeded from the exterior to the interior. The invasion of pathogens triggers the immune response of fish and causes tissue damage to the host. The quantitative real-time PCR results displayed an increase in the relative expression level of immune genes (NK-lysin, LZM, IgM and IgD). Otherwise, the most notable histopathological changes of the infected spotted sea bass were multifocal necrosis. Findings in this study broaden our understanding of the infection conditions of E. piscicida and its pathogenicity to the spotted sea bass.

In Vivo Imaging Sheds Light on the Susceptibility and Permissivity of Carassius auratus to Cyprinid Herpesvirus 2 According to Developmental Stage

Cyprinid herpesvirus 2 (CyHV-2) is a virus that causes mass mortality in economically important Carassius spp. However, there have been no comprehensive studies into host susceptibility or permissivity with respect to developmental stage, and the major portal of viral entry into the host is still unclear. To help bridge these knowledge gaps, we developed the first ever recombinant strain of CyHV-2 expressing bioluminescent and fluorescent reporter genes. Infection of Carassius auratus hosts with this recombinant by immersion facilitated the exploitation of various in vivo imaging techniques to establish the spatiotemporal aspects of CyHV-2 replication at larval, juvenile, and adult developmental stages. While less susceptible than later developmental stages, larvae were most permissive to CyHV-2 replication, leading to rapid systemic infection and high mortality. Permissivity to CyHV-2 decreased with advancing development, with adults being the least permissive and, thus, also exhibiting the least mortality. Across all developmental stages, the skin was the most susceptible and permissive organ to infection at the earliest sampling points post-infection, indicating that it represents the major portal of entry into these hosts. Collectively these findings provide important fundamental insights into CyHV-2 pathogenesis and epidemiology in Carassius auratus with high relevance to other related economically important virus-host models.

Research-Relevant Background Lesions and Conditions in Common Avian and Aquatic Species

Non-mammalian vertebrates including birds, fish, and amphibians have a long history of contributing to ground-breaking scientific discoveries. Because these species offer several experimental advantages over higher vertebrates and share extensive anatomic and genetic homology with their mammalian counterparts, they remain popular animal models in a variety of fields such as developmental biology, physiology, toxicology, drug discovery, immunology, toxicology, and infectious disease. As with all animal models, familiarity with the anatomy, physiology, and spontaneous diseases of these species is necessary for ensuring animal welfare, as well as accurate interpretation and reporting of study findings. Working with avian and aquatic species can be especially challenging in this respect due to their rich diversity and array of unique adaptations. Here, we provide an overview of the research-relevant anatomic features, non-infectious conditions, and infectious diseases that impact research colonies of birds and aquatic animals, including fish and Xenopus species.

Antibiotic-resistant Enterobacteriaceae from diseased freshwater goldfish

Goldfish farming gained more attention among the ornamental fishes in aquaculture industry. The occurrence of bacterial infections and further antimicrobial treatment lead to the major crisis of antibiotic resistance in aquaculture. We have isolated diverse enterobacteriaceae groups which affect the goldfish and identified their response towards 46 antimicrobials of 15 different classes. Thirteen significant bacterial isolates such as Edwardsiella tarda, Serratia marcescens, Klebsiella aerogenes, Proteus penneri, P. hauseri, Enterobacter cloacae, E. cancerogenus, E. ludwigii, Citrobacter freundii, E. coli, Kluyvera cryocrescens, Plesiomonas shigelloides and Providencia vermicola were recovered from the infected fish with the Shannon-wiener diversity index of 2.556. Multiple antibiotic resistance (MAR) index was found to be maximum for P. penneri (0.87) and minimum for C. freundii and E. cloacae (0.22), highlighting the hyper antibiotic selection pressure in the farm. The minimum concentration of antibiotics required to inhibit most of the resistant isolates was found to be > 256 mcg/ml. All the isolates were susceptible towards ciprofloxacin. Plasmid curing and further AMR tests could reveal the location of antibiotic resistance genes mainly as plasmids which determine the large extent of AMR spread through horizontal gene transfer. This study is the first of its kind to investigate the antimicrobial resistance profile of enterobacteriaceae recovered from goldfish, before and after plasmid curing.

Sex chromosome and sex locus characterization in goldfish, Carassius auratus (Linnaeus, 1758)

BACKGROUND

Goldfish is an important model for various areas of research, including neural development and behavior and a species of significant importance in aquaculture, especially as an ornamental species. It has a male heterogametic (XX/XY) sex determination system that relies on both genetic and environmental factors, with high temperatures being able to produce female-to-male sex reversal. Little, however, is currently known on the molecular basis of genetic sex determination in this important cyprinid model. Here we used sequencing approaches to better characterize sex determination and sex-chromosomes in an experimental strain of goldfish.

RESULTS

Our results confirmed that sex determination in goldfish is a mix of environmental and genetic factors and that its sex determination system is male heterogametic (XX/XY). Using reduced representation (RAD-seq) and whole genome (pool-seq) approaches, we characterized sex-linked polymorphisms and developed male specific genetic markers. These male specific markers were used to distinguish sex-reversed XX neomales from XY males and to demonstrate that XX female-to-male sex reversal could even occur at a relatively low rearing temperature (18 °C), for which sex reversal has been previously shown to be close to zero. We also characterized a relatively large non-recombining region (~ 11.7 Mb) on goldfish linkage group 22 (LG22) that contained a high-density of male-biased genetic polymorphisms. This large LG22 region harbors 373 genes, including a single candidate as a potential master sex gene, i.e., the anti-Mullerian hormone gene (amh). However, no sex-linked polymorphisms were detected in the coding DNA sequence of the goldfish amh gene.

CONCLUSIONS

These results show that our goldfish strain has a relatively large sex locus on LG22, which is likely the Y chromosome of this experimental population. The presence of a few XX males even at low temperature also suggests that other environmental factors in addition to temperature could trigger female-to-male sex reversal. Finally, we also developed sex-linked genetic markers, which will be important tools for future research on sex determination in our experimental goldfish population. However, additional work would be needed to explore whether this sex locus is conserved in other populations of goldfish.

Transcription profiles of skin and head kidney from goldfish suffering hemorrhagic septicemia with an emphasis on the TLR signaling pathway

Hemorrhagic septicemia is an acute, highly fatal disease that affects goldfish (Carassius auratus). To gain a better understanding of related immune genes, the transcriptomes of the skin and head kidney of goldfish suffering hemorrhagic septicemia were sequenced, assembled, and characterized. Based on functional annotation, an extensive and diverse catalog of expressed genes were identified in both the skin and head kidney. As two different organs, pair-wise comparison identified 122/77 unigenes up/down-regulated (two-fold change with P<0.05) in the skin and head kidney. Most genes of the immune pathways were expressed and isolated in both skin and head kidney, including interferon (IFN) transcription factors 1-10 and Toll-like receptors (TLRs). Interferon regulatory factor 3 (IRF3), a key IFN transcription factor, was up-regulated at the transcriptional level by polyriboinosinic: polyribocytidylic acid (poly I:C) challenge and regulated the IFN response by increasing the activity of IFN-β and IFN-stimulated response element (ISRE)-containing promoter. This study will benefit the identification and understanding of novel genes that play important roles in the immunological reactions of fish suffering from hemorrhagic septicemia.